recurse: ; @$(MAKE) --no-print-directory -f work/features/parallelism.mk.2 INC=yes all

all: 1 2; @echo success

INC = no
ifeq ($(INC),yes)
-include 1.inc 2.inc
endif

1.inc: ; @echo ONE.inc; sleep 2; echo TWO.inc; echo "1: ; @echo ONE; sleep 2; echo TWO" > $@
2.inc: ; @sleep 1; echo THREE.inc; echo "2: ; @sleep 1; echo THREE" > $@
